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Monday, September 28, 2009

The latest edition of the Carnival of Space, the weekly roundup of the best in the Space and Astronomy blogosphere, is now online over at Cumbrian Sky. Not surprisingly, many of the submitted posts were related to the discovery of water molecules on the Moon, mine included.

Thursday, September 24, 2009

Updated 09/24/2009 12:03 PM MST based on info from this morning's press conference:

The internet is abuzz this evening regarding the possible discovery of wide-spread "water" or hydroxyl molecules of the surface of Earth's moon, a discovery made by spectrometers on three different spacecraft: M3 on Chandrayaan 1, VIMS on Cassini, and Deep Impact. The papers, if I understand correctly, will be published later today in this week's issue of the journal Science and I have not had a chance to look at them. There will also be a press conference later today at 2pm EDT (11am MST) discussing these results.

Why do I bring these results up here on this blog? Well, according the few reports I have been able to find online, like this one here from Universe Today, from Bad Astronomy, and from NASA Watch, this discovery was made by finding a weak absorption band near 3 microns, associated with water and the hydroxyl ion (OH-), concentrated mostly near the moon's high latitude. The absorption band found on the Moon is very weak, suggesting a very low concentration of water or OH- in the moon's soil. The M3 instrument team suggests a concentration of as much as 770 ppm has been observed on the sunlit side of the Moon, according to the NASA Watch posting. While the discovery isn't quite Moon-shattering, previously water ice (or hydrogen anyway) had only been observed within cold-traps in permanently-shadowed craters near the poles.

A similar absorption band was found on Io using ground-based spectroscopy (Salama et al. 1990) and Galileo NIMS (Carlson et al. 1997 and Cataldo 1999) observations. In these measurements, a weak absorption band in Io's near-infrared spectrum at 3.15 microns was observed to be ubiquitous across its surface with a concentration of 4 ppm according to Carlson et al. 1997 and 1000 ppm according to Salama et al. 1990, on the order with what has been observed on the Moon. This absorption band is associated with the O-H stretch transition. Such an atomic bond between an oxygen and hydrogen atom would be found in water, hydrated minerals, or the hydroxyl ion (OH-). Small concentrations of this band have also been observed. An absorption band near 3 microns, attributed to water ice crystals or hydrates mixed with sulfur dioxide frosts, was seen to the north and west of Gish Bar Patera by the Galileo NIMS instrument during the October 2001 I32 encounter (Douté et al. 2004). The low spectral resolution of NIMS at the time (12 spectral measurements spread out between 1 and 5 microns) makes this result a bit tenuous, but if true would indicate that concentrations of possible water ice on top of the low background levels exist on the surface of Io.

So where does the "water" come from on these two, supposedly dry worlds? For the Moon, two possible mechanisms are likely. The first would be recent cometary impacts, which would bring their water to the Moon's surface near the site of these impacts. Concentrations within the ejecta blankets of several small craters on the moon provide further evidence for this hypothesis, but the pattern of the hydroxyl absorption within the ejecta seems to be more consistent with material from the target body rather than material from the impactor. The widespread distribution of water or hydroxyl ions across the moon's sunlit surface suggests another explanation. In this scenario, charged particles, in the form of hydrogen ions and transported from the Sun by the solar wind, impact the Moon's unprotected surface (remember that the Moon is outside Earth's magnetic field most of the time). These hydrogen ions split oxygen atoms from silicate molecules in the Moon's soil, and combine with newly freed oxygen ions to form hydroxyl ions or water. As the day progresses and the Moon's surface heats up, these new molecules themselves split up, freeing the hydrogen to space and returning the oxygen to the soil. The process of water formation from the combination of hydrogen from the solar wind and oxygen in the lunar soil kicks back up the surface starts to cool down in the late afternoon and evening. Alternatively, the water molecules may become excited and be transported to Moon's polar regions, where they are deposited within those aforementioned cold-traps.

For Io, the solar wind can't reach its surface due to Jupiter's strong magnetic field. So where does its water come from? Again, oblique cometary impacts could be a source of water for Io. The two recent cometary impacts on Jupiter in 1994 and again this year would suggest that Io could be hit by water-rich cometary bodies on a regular basis. This could certainly be the source for the concentration found near Gish Bar Patera. For the global ubiquitous concentrations of water or hydroxyl ions, another mechanism maybe necessary. For example, low concentrations of water might be present in Io's magma, like here on Earth. Water vapor would then be released during volcanic eruptions and water ice would be deposited on the surface, however, no water vapor has ever been observed within Io's plumes. Another possibility could be that hydrogen ions from Jupiter's magnetic field break off oxygen from sulfur dioxide and silicate compounds on the surface then combine with them to form OH- or water, akin to the preferred scenario for the Moon.

This discovery of OH molecules on the Moon is certainly interesting, and just goes to show everyone that water is quite common place in the solar system, even in the driest of places.

Sunday, September 20, 2009

While normally I remember to cover the big four conferences for planetary science each year (LPSC in March, AGU in May and December, and DPS in the fall), I often forget about the main European planetary science conference, the European Planetary Science Congress or EPSC. Io science tends to be dominated by American institutions like the University of Arizona, the Jet Propulsion Laboratory, and Arizona State University, so a European conference would be expected to have less Io coverage than those held in the US. However, this year, three talks and one poster were presented last week in Potsdam, Germany. Let's take a look at the abstract for these four presentations:

The last talk in the Satellite Atmospheres session this past Thursday, September 17, was titled, "First detection of Io's atmosphere at 4.0 micron" and was presented by Emmanuel Lellouch et al. Lellouch and his colleagues observed Io in the near-infrared using the CRIRES spectrometer at the Very Large Telescope in Chile in July 2008. These measurements allowed the authors to observe an absorption band of sulfur dioxide gas at 4.0 μm. With the adaptive optics system at VLT, they were also able to spatial resolve variations in the absorption band, looking for differences in atmospheric column density between the polar region and the equator. Lellouch et al. believe that these observation open up a new avenue for monitoring Io's dynamic atmosphere.

The Satellite Surfaces and Interiors oral session this past Wednesday, September 16 hosted two Io talks. The first was titled, "Volcanism on Io: New Insights from Global Geologic Mapping" and was presented by David Williams et al. This talk and abstract provide an update to the Io geologic mapping project, a subject Williams presented at this year's Lunar and Planetary Sciences Conference, which I discussed in greater depth earlier this year. This geologic map displays the distribution of various morphologic and color units across Io's surface. Using software such as ArcGIS™ will allow researchers to use the map to conduct various lines of research, including comparing the areas of various mountain units with their heights, looking at the areal extent of the various plains units, and seeing how ongoing volcanism change these areal extent of the different lava flow units. The authors also plan to assess the distribution of different flow units to assess regional variations in the style of volcanism (sulfur versus silicate volcanism, for example) and compare these units to observed volcanic hotspots to look for correlations between eruption style and unit types. The Io Geologic map was completed in February 2009 and has been submitted to the USGS for peer review. A similar global geologic map of Ganymede was also presented at the conference, which has garnered some press coverage (but not Io's, bah I say, BAH!) .

The other Io talk at the Surfaces session was titled, "Continued Observations of Io's Volcanic Activity" and was presented by Imke de Pater et al. In this abstract, de Pater briefly presents new results from observations of Io in the near-infrared using the adaptive optics system at Keck in Hawaii. These results include new observations of volcanic hot spots on Io as well as the distribution of sulfur dioxide frost across Io's surface. While the abstract left out specifics, the authors spent more time advocating for additional telescopic observations of Io. Regular observation runs were conducted during the Galileo mission and the New Horizons flyby in 2007, but outside of that flyby, Io monitoring has been sparse the last few years. Regular monitoring is important for understanding Io's heat flow and its active volcanism. de Pater et al. also advocate for the inclusion of narrow angle cameras on board Jupiter-bound missions, particularly EJSM, to provide for monitoring of Io and other satellites in the system.

Finally, Ashley Davies, Laszlo Keszthelyi, and Alfred McEwen presented a poster titled, "Determining Io Lava Eruption Temperature: Strategies for a New Mission to the Solar System's Most Dynamic Satellite." These authors presented a similar poster at LPSC earlier this year, which was discussed here a bit more extensively. This abstract discusses how the desire to measure the temperature of Ionian lava using near-infrared camera observations of lava fountains and skylights (holes in the roofs of lava tubes). The authors explain that near-simultaneous, high-resolution color imaging during Io flybys by either a dedicated Io mission (such as the Io Volcano Observer) or by another Jupiter system mission (like EJSM) would be necessary for determining these lava temperatures without the issues from short-term variability (on the order of a few seconds) of lava fountains. The authors also state that these observations would need to be preformed over Io's night-side to avoid contamination from sunlight.

Saturday, September 19, 2009

This week's Carnival of Space, the 121st of its name, has been posted over at the blog Next Big Future. Check it out to read the best posts this week in the space blogosphere. Read about new results concerning Jupiter's aurorae, from the Lunar Reconnaissance Orbiter, and a treatise on spotting the International Space Station.

Speaking of the latest in the space blogosphere, I direct your attention to one new blog and another reactivated blog. The former is Dr. Schenk's 3D House of Satellites, where new stereo images and movies will be posted based on data from Voyager, Galileo, and Cassini. His last few posts include movies showing the topography of Conamara Chaos on Europa, the Uranian moon Miranda, and the south polar region of Enceladus using images acquired during two encounters last year. Schenk promises to add stereo views of the Callanish impact crater on Europa as well as some views of Io, though that will take a longer to prepare due to Io's funky phase functions and surface changes.

The final draft of the Io Decadal Survey White Paper has been posted online. The white paper consists of two sections: the first summarizes the state of Io science, the justification for NASA sending additional missions, and the outstanding questions that should be addressed by future exploration of the satellite; the second discusses an exploration strategy for addressing these remaining questions. The other submitted white papers can be found on the National Academies website; Van Kane has a good summary on his blog of these other papers. I previously posted a note about the recommendations for future missions to Io based on an earlier draft of the white paper.

Let's take a look at the two Io white papers. The first, Justification and Science Objectives, takes a look at the reasons why other planetary scientists should be interested in exploring Io, the outstanding questions left by the exploration of Io by Galileo and New Horizons, and the science objectives that a future Io mission or series of Io missions should attempt to accomplish. In addition to the fact that Io is just plain awesome and everyone knows it ("Finally, as one of the most spectacular places in the Solar System, Io has unique public
appeal, and Io exploration offers many opportunities to attract and engage public interest in
planetary science."), the authors point out that studying Io provides opportunities to understand processes that are important to examine in general, including: satellite-magnetosphere interactions; the mechanics of tidal heating, an important process for Io as well as Europa, Ganymede, and Enceladus, as well as for extra-solar planetary systems; volcanism, particularly that found on the Moon and Archean Eon Earth; and the dynamics of thin atmospheres, particularly those which are strongly driven by surface temperature and vapor pressure. The authors also identified eight science objectives that an Io exploration campaign would attempt to accomplish (the sub-headings are my own notes):

Determine the magnitude, spatial distribution, temporal variability, and dissipation
mechanisms of Io’s tidal heating. (We would like to add “and implications for the coupled
orbital-thermal evolution of Io and Europa.”)

The latter goal can be helped by the examination of Europa to be performed by EJSM.

This fits into the previous objective. As explained later in Part 1, understanding the state of the core, its Fe/S ratio, and its size would help us understand the result obtained by Galileo that suggests that Io does not have a magnetic field. Resolving the conundrum of why Io can be so active and not have one might help us better understand how planetary magnetosphere are created.

Understand the eruption mechanisms for Io’s lavas and plumes and their implications for
volcanic processes on Earth, especially early in Earth’s history when its heat flow was
similar to Io’s, and elsewhere in the solar system.

Two good places to provide comparative studies would be the Moon and Mercury. While these two worlds are dead as a doornail now (deader actually), earlier in their histories, they experienced volcanic eruptions similar to those we see on Io now, particularly flood basalt eruptions and pyroclastic flows.

Investigate the processes that form Io’s mountains and the implications for tectonics under
high-heat-flow conditions that may have existed early in the history of other planets.

Understand Io’s surface chemistry, including volatiles and silicates, and derive magma
compositions (and ranges thereof), crustal and mantle compositions and implications for the
extent of differentiation, and contributions to the atmosphere, magnetosphere, and torus.

Understand the composition, structure, and thermal structure of Io’s atmosphere and
ionosphere, the dominant mechanisms of mass loss, and the connection to Io’s volcanism.

Investigate the neutral and plasma densities and energy flows in the Io plasma torus, plus their
variations over time, and characterize the ionic radiation belts in the vicinity of Io and their
influence on the surface.

The second part of the Io white paper, Recommendation for Missions, was more extensively discussed in my last post on this subject. To answer Ted's comment for that post, where he suggested that IVO, a proposed Discovery-class Io mission, would be the most likely to fly, keep in mind that what is discussed in Part 2 is an exploration program, akin to what is currently going on for Mars. For Io, this program would start with either a New Frontiers- or Discovery-class mission that would orbit Jupiter and flyby Io on several occasions. Such a mission could be flown in the 2013-2023 decade covered by this survey. Following this "Io Observer" mission, the next decade, 2023-2033, could see a follow-on mission that would orbit Io, providing detailed global maps using UV, visible, and near-infrared imagers and a laser altimeter, as well as measuring Io's gravity and possible magnetic fields, and deploy one or more in situ components, such as penetrators, landers, or rovers. One important task for these in situ missions would be to measure seismic activity using seismometers. Enough activity should be detected over a period as short as a day to provide a more detailed model of Io's interior structure. Finally, the authors support telescopic observations of Io from Earth or from space-based platforms, including a UV telescope that would replace the capabilities that will be lost once Hubble is de-orbited and additional ground-based telescopes with adaptive optics capabilities, which would help ease scheduling pressures at telescopes such as Keck II. These observations would allow for monitoring time-variable phenomenon at Io such as satellite-magnetosphere interactions, Io's atmosphere, and its volcanic activity.

Additional white papers can be found at the National Academies website as well as summaries at Van Kane's blog. These white papers will be used as input into the upcoming Decadal Survey report, which will outline the direction planetary science should go within NASA over the next decade. How much will be possible is up in the air as the planetary budget is projected to remain pretty flat over the next decade. For Io, since the recommendations call for a fairly modest program over the next decade (one mission in either the New Frontiers or Discovery programs) with the major mission to be started in the decade following, it isn't impossible that such a program could fly. What remains to be seen is how much NASA and the community will take to heart the first suggestion made by the authors of the Io white paper:

We recommend that NASA pursue a balanced solar system exploration program between life-focused and physical-science focused missions.

Friday, September 18, 2009

The science program as well as the abstracts for this year's DPS meeting were posted online a few weeks ago. DPS 2009 will be taking place in Fajardo, Puerto Rico, and as such, I won't be going. Hopefully the organizers will be able to broadcast the meeting oral sessions like they did for last year's meeting, but I guess that will depend on the infrastructure at the El Conquistador Resort. The webcasts last year allowed me to post about each of the Io talks here on this blog.

The 2009 Meeting of the Division of Planetary Sciences will be held between October 4 and 9, 2009.

In this year's science program, there are four Io talks and one poster planned. All four Io talks will be held during the Galilean Satellites oral session on the afternoon of Friday, October 9. The Io-related poster will be in the Decadal Survey White Papers section during the poster session on the evening of Tuesday, October 6. Here is a brief summary of the talks and posters to be presented:

The next talk will be by Daniel Allen and Jani Radebaugh and is titled, "Temperature and Variability of Three Ionian Volcanoes." The authors examined the brightness and color temperature of three volcanoes on Io (Pillan, Wayland, and Loki), observed by Cassini's narrow-angle camera during several eclipses during the New Year's 2001 Cassini encounter. The abstract and talk provide an update to the results presented at this year's Lunar and Planetary Sciences Conference back in March, including updated temperature estimates for the three volcanoes. They found a cooling trend for Pillan and Wayland, suggestive of a cooling lava flow, as well as an apparent shut down of a vent at Wayland during one of the eclipses. Loki was found to be more variable, consistent with an active lava lake.

John Spencer et al. will present the next talk in the Galilean satellites session, titled, "Changes in Io's Atmosphere in 2009: Atmospheric Inflation Near Perihelion?". During his talk, Spencer will present spectroscopic observations of Io's atmosphere acquired in June 2009 which suggests that Io's atmosphere undergoes an increase in column density (the amount of gas over a given area on Io's surface) as the Jupiter system approaches perihelion. This would be the result of increased sublimation of sulfur dioxide frost from Io's surface from the increased surface temperatures. The vapor-pressure equilibrium relationship between SO2 surface frost and atmosphere SO2 gas is sensitive enough that even the 5.6 K temperature difference between aphelion and perihelion is expected to produce a 7x difference in atmospheric SO2 vapor pressure. This data was also discussed in more detail by John Spencer in June in the Planetary Society Blog.

Thursday, September 17, 2009

Last week, a new paper was published in press (the paper has been approved for publication, but hasn't found a slot in the dead-tree version of the journal yet) in the journal Icarus discussing spectroscopic observation of Io as it emerged from the shadow of Jupiter. The paper is titled "Eclipse reappearances of Io: Time-resolved spectroscopy" and was written by Dale Cruikshank, Josh Emery, Katherine Kornei, Giancarlo Bellucci, and Emiliano d'Aversa.

In this new paper, the authors discuss spectroscopic observation of Io acquired using NASA's Infrared Telescope Facility (IRTF) in Hawaii during five eclipse reappearances in April, May, and June 2004. These observations were intended as a follow-up to results from Cassini VIMS observations in Bellucci et al. 2004 taken during that spacecraft's Jupiter flyby during New Year's 2001 that showed a brightening of Io's surface in the near-infrared and a deepening of several strong sulfur dioxide absorption bands following Io's emergence from Jupiter's shadow. This result continues a 40-year-long mystery concerning the interaction between Io's atmosphere and its surface during and after an eclipse by Jupiter.

Unlike lunar eclipses, when the Earth passes between the Sun and our Moon and which happen about once a year, or every 13 orbits of the Moon around the Earth, eclipses of Io by Jupiter occur about once each Ionian day. This is due the large size of Jupiter compared to Earth and the much lower axial tilt of Jupiter and its main satellite system. Each Ionian lunar eclipse lasts about 2 hours and 22 minutes. During this time, the temperature of Io's surface cools due to the sudden lack of sunlight. As Io cools down as the eclipse progresses, atmospheric Sulfur dioxide (SO2) condenses onto the surface. Check out a post I wrote earlier this year on another paper for more details on this process.

Depending on the amount of SO2 that condenses onto the surface, the fresh frost should be visible shortly after Io emerges from behind Jupiter's shadow as a brightening of Io's surface compared to its appearance prior to being eclipsed, and it should quickly dim as the frost sublimates from the surface now that the Sun is able to heat it up. In addition, the strong SO2 absorption bands at 3.56 μm, 3.78 μm, 4.07 μm, and 4.37 μm would be deeper than they were prior to the eclipse and should become shallower during the first 60-90 minutes after each eclipse and particularly in the first 15 minutes as the fresh, fine-grained SO2 frost sublimates back into the atmosphere. Results from multiple studies using ground-based and spacecraft observations over the last 40 years, since Binder and Cruikshank 1964 revealed a brightening of Io of 10 percent following an eclipse by Jupiter, have been inconsistent with some showing such a brightening, and others showing none. As explained in this new paper, Nelson et al. 1993 found that post-eclipse brightenings are likely to be rare as a fresh SO2 frost layer several millimeters thick would be required to explaining the magnitude of the brightenings that were seen, and it would take longer than 15 minutes to sublimate that layer away. In addition, modeling of Io's atmosphere during an Io eclipse by Moore et al. 2009 suggests that SO2 condensation onto the surface would be curtailed to some degree by atmospheric heating by the Io plasma torus and by non-condensable species like Sulfur monoxide preventing SO2 in Io's upper atmosphere from condensing.

Cruikshank et al. examined their observations taken at IRTF and found no evidence of changes in Io's albedo or the area of three SO2 absorption bands at 3.56 μm, 3.78 μm, and 4.07 μm. What changes were observed were either the result of the rotation of Io during the 60-90 minutes of each observation run, were found in one absorption band but not in the other three, or were the result of observation noise or the thick airmass of Earth's atmosphere. Therefore, the authors were not able to confirm the VIMS results published by Bellucci et al. 2004. The authors suggested that the two conflicting results could be due to the background frost coverage in the area observed by the two groups of researchers. VIMS observed Io's trailing hemisphere which is thought to have the least abundant SO2 frost coverage while the Cruikshank et al. group observed the sub-Jupiter hemisphere, SO2 abundance is higher. The lower SO2 abundance would have made condensed SO2, even if in a very thin layer, more noticeable compared to the sub-Jupiter hemisphere.

In other results, Cruikshank et al. observed additional SO2 absorption bands between 2.11 and 2.24 μm, including a faint one at 2.198 μm that the authors thought they were first to see in Io's near-infrared spectrum. Another weak absorption band at 2.1255 μm was mapped by Laver and de Pater and the results of that study were published earlier this year and discussed on this blog. Cruikshank et al. also observed Io's emission spectrum while the satellite was still in the shadow of Jupiter during the observation run on June 22, 2004. They did not find convincing evidence for condensed SO2 in Io's atmosphere, which would be expected in Io's volcanic plumes. This negative result could be the result of the temporal variability of Io's plumes.

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I work for the Cassini Imaging team, usually processing Titan and Enceladus images and making maps of Titan based on our images. When I am not working or studying, I'm...I forget. I watch a lot of movies I guess.